The most popular flat panel display technology currently in use is based on liquid crystal devices, which are effectively light shutters used in combination with illumination sources. In graphic displays there are many different pixels that must be independently driven. Typically this is achieved through matrix addressing, where each pixel is addressed by application of a suitable switching voltage applied between row and column conducting tracks on either side of the liquid crystal. Each row is selected by applying a voltage to the row track, and individual pixels within the row are selected by application of column data voltages to the column tracks. The rows are addressed sequentially, each for a line address time such that the whole frame is addressed within the frame time. However, because the speed of switching of the liquid crystals is slow relative to the line addressing time, when video frame rates are required (&lt;20 ms), special circuitry has to be added to each pixel. This arrangement is called active matrix addressing and often involves the use of thin-film transistors at each pixel. Because of the increased complexity of the active matrix displays they are much more expensive to make than passive matrix devices.
Electroluminescent devices are made from a layer of a suitable material between two conductive electrodes. The material emits light when a suitable voltage is applied across the electrodes. One class of such materials is semiconductive conjugated polymers which have been described in our earlier Patent U.S. Pat. No. 5,247,190, the contents of which are herein incorporated by reference. The electrodes can be patterned to form a matrix of rows and columns so that matrix addressing can take place. There are several potential advantages over liquid crystal graphic displays. Because the polymers are directly emissive, no backlight is required. Also polymers of different colours can be fabricated so that a suitably patterned matrix of polymers can be used for a colour display without the use of colour filters as required by a liquid crystal display. Perhaps most significantly, the light emitting polymers are very fast, easily achieving switching times of 1 microsecond, and therefore they are able to react directly when a particular row is selected. Unfortunately, when the row voltage is removed they immediately switch off. To achieve a given average brightness for the display as a whole, each individual line needs to be driven at a peak brightness that is higher by a factor L, where L is the number of lines. The peak brightness that a given emitting area can achieve is limited by the amount of current that can be injected into the semiconductor due to space charge effects.
So-called thin film inorganic electroluminescent devices are also known, as described for example by M. J. Russ and D. I. Kennedy in the Journal of the Electrochemical Society, vol. 114 (1967) page 1066, whose contents are herein incorporated by reference. These too can suffer from the same problem. Phosphor materials are sandwiched between dielectric layers and conducting electrodes, and high ac fields are applied across the structure. When used in displays with a matrix addressing scheme, the average luminance of the display decreases with the number of lines due to a limitation in current densities. One way that this problem has been tackled is by the use of a photoconductor layer integrated with the device (e.g. an amorphous silicon layer deposited between one of the electrodes and the normally adjoining dielectric layer) as described by P. Thioulouse and I. Solomon in IEEE Transactions on Electron Devices, vol. ED-33, (1986), page 1149. The photoconductor layer provides a "memory effect" which allows a device to be turned on and driven with a given light output; subsequently the voltage can be reduced without a reduction in light output, but with the new voltage still below the original turn-on threshold voltage.